What does "black" mean?

Ever since Rebecca learned her colors a few months ago, she would comment on whatever colors she happened to see, especially when we were driving in the car, which meant she said "red" and "green" annoyingly frequently. So we decided to teach her that "red" means "stop" and "green" means "go." Then she would say "red means stop" and "green means go" whenever she saw anything of those colors, not just traffic lights. Then Uncle Stephen taught her that "yellow means slow," although it doesn't always mean that where we live. Of course, it wasn't long before she wanted to know what all the other colors mean. So now, in addition to traffic light colors, we have:

• blue means information
• orange means caution
• purple means royal
• white means pure, and
• pink means pretty

Now, the other colors besides pink are arguably pretty, too, but we couldn't think of anything else to distinguish pink, so that's what we came up with. So now I'm trying to decide what "black" represents, whenever Rebecca asks me that one. My first thought is that it represents sin, but I'm not sure I want Rebecca saying to people whenever they wear black, "Black means sin!" Any ideas on what to tell her?